Okay, I've tried to get some diagrams together...can't say my graphic skills are up to the standards of this thread yet, but maybe I'll get there.

Meanwhile, I hope they convey what I mean.
Situation 1: A Huge monster controls/needs a space of 15'x15'. That is translated to 3 squares across in the D&D measurement of 5 feet = 1 square = 1 inch. Hence, it can function nicely in a corridor of 15 feet width like this.
But so can it in a corridor angled at 45°. The 1-2-1-2 rule states that a diagonal is alternatively equal to 1 or 2 squares across, or 1.5 squares on average. This means the 15'x15' area of the Huge monster doesn't change size when you place it into the angled corridor. As you can see on the second diagram, the area it occupies stays the same, and the 15 feet border turn into 2 diagonals...2 diagonals equalling 1+2=3 squares, which is the battlemat translation of 15 feet. The corridor is the same 15' width all over the diagram (you hopefully will excuse the inexactness due to my PC drawing improficiency, I'm better with a ruler and a pen), and the monster's area of combat has the same size as before, as demonstrated by the constant number of squares it fills. By the way, the red squares were left in as a measurement of area size, namely the area a properly sized, square mini base would cover on the map, and as nothing else.
I hope I was able to illustrate my point sufficiently

. If not...well, I guess I need to hunt down some monster tiles from old Dungeon magazines and create a little battlemap, and really take pictures. It won't change the fact that monsters (and their combat areas) don't change size when turned at 45° to the grid, because the 1-2-1-2 rule conforms to the same physical reality (with an error margin of 6%, granted) that the minis/tiles on the battlemap do.